1. Field of the Invention
The present invention relates to an optical sampling waveform measuring apparatus which measures an optical wavelength in an ultrashort time period which cannot be measured by a method using a photoelectric conversion element. The present invention particularly relates to an optical sampling waveform measuring apparatus which can measure an optical waveform of an input signal light to be measured with high sensitivity and with high time resolution.
2. Description of Related Art
In an ordinary optical sampling waveform measuring apparatus, a sum frequency light (hereinafter called SF light) by which a formula such as ω3=ω1+ω2 is effective by performing cross co-relation of a light pulse to be measured (angular frequency ω1) in a nonlinear optical crystal and a sampling light pulse having a narrower pulse width than that of the light pulse (angular frequency ω2) to be measured has been extracted (See Japanese Examined Patent Application, Second Publication No. 6-63869).
The above-mentioned optical sampling waveform measuring apparatus performs photoelectric conversion of the SF light which is obtained by the nonlinear optical effect by the light receptor in the nonlinear optical crystal and displays the waveform by processing signals electrically.
In the optical sampling waveform measuring apparatus, the time resolution in the measurement of optical waveform is limited by the pulse width of the sampling light pulse and group velocity delay between the sampling light pulse and signal light to be measured.
Also, nonlinear optical effect is used for measuring an optical waveform, it is necessary to perform phase matching for generating an SF light.
For a method of phase matching, an angle matching method can be mentioned in which the incident angle of sampling light pulse and signal light to be measured into a nonlinear optical crystal element is adjusted by using birefringence of the nonlinear optical crystal such that phase matching of sampling light pulse, signal light to be measured, and SF light to be generated are realized.
Here, a first kind of phase matching condition is defined as a phase matching condition under which a sampling light pulse and a signal light to be measured are incident in the same linear polarization direction. A second kind of phase matching condition is defined as a phase matching condition under which a sampling light pulse and a signal light to be measured are incident in an orthogonal linear polarization direction.
At present, for the nonlinear optical crystal which has been used for generating nonlinear optical effects most commonly, KTP (KTiOPO4) using the second kind of phase matching condition can be named (See reference document 1: Development of 310 GHz optical sampling system, by Kawaguchi, Nogiwa, Ota, and Endo, The Institute of Electronics, Information and Communication Engineers, Society Meeting, B-10-149, 2000).
However, the nonlinear optical effect of KTP crystal under the above-mentioned second kind of phase matching condition is not sufficient, and it is not possible to improve the measuring accuracy of a waveform of the signal light to be measured.
On the other hand, in the first kind of phase matching condition in which a sampling light pulse and a signal light to be measured are in the same polarization state, if a nonlinear optical crystal which can realize a nonlinear optical effect is used, it is possible to improve waveform measurement accuracy by using a large nonlinear optical constant in a nonlinear optical crystal.
However, KTP crystal and a LiNbO3 having a large nonlinear optical constant do not have the first kind of phase matching condition; therefore, it is not possible to use the large nonlinear optical constant.
Recently, an idea of pseudo-phase matching is proposed by which a phase matching condition is realized artificially by inverting polarization of the nonlinear optical crystal periodically such that the first kind of phase matching condition is satisfied.
According to the pseudo-phase matching, the polarization of the nonlinear optical crystal is inverted by charging a high electric field at a high temperature under conditions in which the nonlinear optical crystal which is used in the nonlinear optical element is an LiNbO3, both wavelength of the sampling light pulse and wavelength of the signal light to be measured are in nearly 1550 nm and the frequency of polarization of the above-mentioned nonlinear optical crystal is nearly in 18 μm; thus, the periodically domain-inverted LiNbO3 crystal (PPLN: Periodically-polled Lithium Niobate) can be generated. The first kind of phase matching condition can be realized artificially when the sampling light pulse and the signal light to be measured are incident in the PPLN; therefore, a large nonlinear optical effect can be obtained.
The optical sampling waveform measuring apparatus using such a theory has been reported (See reference document 2, “Development of high sensitive optical sampling system using PPLN crystal” by Nogiwa, Kawaguchi, Ota, Endo, The Institute of Electronics, Information and Communication Engineers, General Meeting, B-10-170, 2000).
The photoelectric inversion of the SF light which is obtained by using the above-mentioned first kind of phase matching condition is performed by a receptor such as a photoelectric inversion element, a waveform of the sampling result is displayed by processing an electric signal, and characteristics of the signal light to be measured are evaluated. A structure of a conventional optical sampling waveform measuring apparatus is shown in FIG. 7.
In FIG. 7, an electric signal generator SG1 generates, for example a periodical electric signal and outputs an electric signal P1 having frequency fsig as a cycle period.
An electric signal generator SG2 generates, for example a periodical electric signal and generates an electric signal P2 having frequency ((fsig/n)−Δf) as a cycle period which synchronizes the electric signal P1.
An amplifier 100 amplifies the input sampling pulse signal P2 and gains a narrow width electric pulse by an narrow pulse generator 101.
A laser oscillator 102 generates a narrow width light pulse by a gain switching method by using the electric pulse. An optical circulator 103 inputs a continuous light (CW light) which is generated by a laser oscillator 104 to the laser oscillator 102 in order to reduce timing jitter of the sampling light pulse and outputs the light pulse P3 which is generated by the laser oscillator 102.
A DCF (Dispersion Compensation Fiber) 105 performs a linear compression of the above-mentioned light pulse P3. An EDFA (erbium-doped fiber amplifier) 106 amplifies the linearly-compressed light pulse P3. A DSF (Dispersion shift fiber) 107 extends the input light pulse P3 to a rectangular shape.
Next, an optical amplifier 108 amplifies the light pulse P3 which is transformed into a rectangular shape and performs pulse compression of the light pulse P3. A polarization direction controller 109 controls a polarization direction of the light pulse P3 and outputs a sampling light pulse P4.
An MLFRL (mode-locked fiber ring laser) 110 generates a light pulse P6 which synchronizes the frequency of the signal light P1 to be measured.
A light intensity modulator 112 modulates the light pulse P6 by using a predetermined pattern (data row made from “0 (zero)” and “1 (one)”) which is output by a pattern generator 111 synchronizing the signal light P1 to be measured and outputs the modulated light pulse P7.
A light amplifier 113 amplifies the light pulse P7. A polarization controller 114 controls the polarization direction of the input light pulse P7 and outputs light pulse P8.
A wavelength division multiplexer 115 mixes the light pulse P8 and the sampling light pulse P4 and outputs the multiplexed light pulse P9.
A PPLN 116 is a nonlinear optical crystal element. A PPLN 116 is also a periodically domain-inverted nonlinear optical crystal as explained above and satisfies the first kind of phase matching condition artificially.
When the phase matching of the sampling light pulse P4 and the light pulse P8 is completed under the first kind of phase matching condition, the PPLN 116 ejects the SF light of the light pulse P9 which is a cross co-relation signal of two light pulses as a result of the large nonlinear optical effect.
Here, the polarization directions of both incident lights are the same; therefore, the phase matching occurs when both lights are extraordinary lights or when both lights are ordinary lights due to the birefringence of the nonlinear optical crystal.
A light band pass filter 117 removes a noise component from of the SF light and outputs the above-mentioned SF light which is a light pulse having only angular frequency (ω1+ω2).
A receptor 118 is a photoelectric inversion element such as an avalanche photodiode and performs photoelectric inversion of the input SF light and outputs measured signal PS.
An A/D inverter 119 inverts a peak voltage of the input measured signal PS into a digital value by a predetermined timing and outputs it.
A computer 120 performs the processing of the above-mentioned digital value, generates an optical eye pattern, displays an image of the optical eye pattern, and evaluates characteristics of the waveform of the signal light (light pulse P7) to be measured which is used for communication.
As mentioned above, when the pseudo-phase matching condition using PPLN is used as shown in FIG. 7, an optical sampling waveform measuring apparatus can achieve highly sensitive sampling waveform measuring because of the large nonlinear optical effect.
As explained above, the time resolution depends on the sampling pulse width and the group velocity delay.
According to the first kind of phase matching condition, it is possible to restrain the group velocity delay at one hundredth of the second kind of phase matching condition. For example, the group velocity delay of the 1 cm length of the crystal is several 10s of fs; thus, it is possible to obtain high time resolution.
The above-mentioned pseudo-phase matching has a problem such as an occurrence of a second harmonic beam.
Ordinarily, a sampling light pulse and a signal light to be measured are generated so as to have a wavelength of nearly 1550 nm. The pseudo-phase matching condition is a first kind of phase matching condition; therefore, the second harmonic beam of the sampling light pulse and the signal light to be measured is generated in addition to the SF light
Here, a light pulse having a narrower width than the signal light to be measured is used for the sampling light pulse in order to perform sampling of the signal light to be measured.
The power of the sampling light is so high that the output of the second harmonic beam of the sampling light pulse is greater than the output of the SF light which is generated by the sampling operation.
Also, the wavelength of the second harmonic beam of the SF light and the sampling light pulse become closer to 775 nm. Therefore, it is necessary to select only an SF light by using a light band pass filter (BPF) 117 so as to cut the second harmonic beam.
Here, in a optical sampling waveform measuring apparatus shown in FIG. 7, the wavelength of the SF light and the wavelength of the second harmonic beam are very close; therefore, there is a problem in that the second harmonic beam cannot be removed, or that a transmission loss of the SF light becomes large.
Furthermore, wavelength of the generated SF light changes according to wavelength of the signal light P8 to be measured; therefore, it was necessary to adjust the selected wavelength of the BPF 117 each time the wavelength of the SF light changed. Spectrum of the SF light and spectrum of second harmonic beam overlapped when the second harmonic beam was in a certain value, it was anticipated that a component of the second harmonic beam which passes the BPF 117 increases; thus, it is not possible to remove the second harmonic beam completely.
As a result, in the conventional optical sampling waveform measuring apparatus shown in FIG. 7, there are problems such as inferior sensitivity due to the existence of the second harmonic beam, inferior signal-noise ratio, and the signal light having non-measurable wavelength.
As explained above, the conventional optical sampling waveform measuring apparatus can utilize large nonlinear optical constant so as to measure very sensitively when a first kind of phase matching condition including pseudo-phase matching as a nonlinear optical effect is used.
On the other hand, in the conventional optical sampling waveform measuring apparatus, there are problems such as inferior sensitivity due to the existence of the second harmonic beam, inferior signal-noise ratio, and the signal light having non-measurable wavelength.
Here, the object of the present invention which is established in consideration of the above-mentioned problems is to provide an optical sampling waveform measuring apparatus which can measure waveforms of ultra-high speed signal light to be measured very sensitively, in high time resolution, and very accurately, by using Raman shift light having narrow pulse width according to the signal light to be measured as a sampling light.